The invention relates to the use of LNG in combination with a combined cycle plant (gas turbine plant/steam turbine plant) or a gas turbine plant. The LNG is regasified and chills a heat exchange fluid which fluid is used to cool and densify the intake air for a gas turbine. The heat exchange fluid is then used in one or more heat transfer steps. The regasified LNG is also used as fuel for the gas turbine and optionally for distribution to other power plants and natural gas distribution systems.
It is state of the art practice to extend a gas turbine plant with a waste-heat boiler and to combine the gas turbine plant with a steam turbine plant. The gas turbine and the steam turbine each drive their own generator or drive a single generator via a common shaft. These combination plants, referred to as combined cycle plants, are generally distinguished by their very good conversion efficiencies which range in the order of magnitude from 50 to 52%. These high efficiencies result from the cooperation of a gas turbine with at least one steam turbine plant. The gas turbine exhaust gases are passed through a waste-heat boiler and the residual heat potential of these waste-gases is utilized for producing the steam required for feeding the steam turbine. LNG has been used in combined cycle plants as a combustion energy source.
LNG is normally transported overseas as a cryogenic liquid in specialized vessels. At the receiving terminal this cryogenic liquid, which is approximately at atmospheric pressure and at a temperature of around xe2x88x92260xc2x0 F., has to be regasified and fed to a distribution system at ambient temperature and at a suitably elevated pressure, typically ranging up to 80 atmospheres. The liquid is pumped to the required pressure so that when heat is added and it is regasified, no compression of the resultant natural gas is required.
Although many suggestions have been made and some installations have been built to utilize the large cold potential of the LNG, in most receiving terminals the cold potential is wasted and the LNG is simply heated with a large flow of sea water which has to be applied in such a manner as to avoid ice formation.
At a few terminals, the cold potential is utilized in air separation plants or similar cryogenic installations or for refrigeration purposes in the freezing and storing of foodstuffs. It has also been proposed to use the cold LNG as a heat sink in a power cycle to generate electrical energy. A number of possible cycles have been proposed which seek to overcome the difficulties caused by the large temperature difference through which the LNG is heated and the particular shape of the warming curve. However, it has been found that even with relatively simple cycles only a small part of the available cold potential can be utilized. Proposals to increase the efficiency employ more complex cycles involving a large number of turbines operating between different pressure levels.
U.S. Pat. No. 3,978,663 broadly discloses a method for improving the efficiency of gas turbines by cooling a stream of intake air with LNG. However, the process requires that coolants be mixed with the air to lower the freezing point of separated-out water.
U.S. Pat. No. 4,036,028 also discloses the use of LNG to cool the intake air of a gas turbine but again the coolant must be mixed with the air to prevent freezing of the separated-out water.
U.S. Pat. No. 4,995,234 discloses a power generation system which utilizes high pressure natural gas and high pressure high temperature carbon dioxide to drive turbines. To cool the intake air of a gas turbine, the intake air is placed in direct heat exchange relationship with the natural gas.
In our parent application, the invention broadly embodied a system and process which improved the capacity of a combined cycle plant in an amount up to 9% and the efficiency of the plant up to about 2%, particularly when the ambient temperature exceeded 60xc2x0 F. A LNG fuel supply system was used in combination with the combined cycle plant. A primary heat exchange fluid was chilled, in a two step process, in the LNG fuel supply system and was then utilized in the gas turbine process to cool and densify the intake air to the gas turbine. The primary heat exchange fluid was also utilized in the steam turbine process to condense the spent stream from the steam turbine. Lastly, the primary heat exchange fluid was recycled to the LNG fuel supply system where it was rechilled. The primary heat exchange fluid flowed through a closed loop while cooling and densifying the intake air, while condensing the steam discharged from the steam turbine and when being rechilled in the LNG fuel supply system.
The present application discloses two further alternative embodiments of the invention(s) disclosed in our parent application with the same improvements in capacity 9% and efficiency 2%. The present application embodies the efficacious use of the thermal energy of LNG when the LNG is regasified. A heat exchange fluid is chilled, in a single step, in the LNG fuel supply system which chilled heat exchange fluid initially is used to cool and densify the intake air for a gas turbine. This heat exchange fluid is subsequently used in at least one other heat transfer step in a power generating process before it is recycled and rechilled by the expanding LNG. In one embodiment of the invention, the heat exchange fluid, after cooling and densifying the intake air, flows through a condenser associated with a steam turbine plant and is then subsequently rechilled. In another embodiment of the invention the heat exchange fluid, after cooling and densifying the intake air, flows through a heat recovery heat exchanger and is then subsequently rechilled.
More particularly, in one embodiment of the invention, the heat exchange fluid, a water/glycol mixture, flows through a regasifier/chiller (heat exchanger) in the LNG fuel supply system. This heat exchange fluid then flows through a heat exchanger in the gas turbine plant. The gas turbine plant, which is fueled by the gasified LNG, drives a generator. The gas turbine plant has an air intake duct, the heat exchanger, a water separator, an air compressor, a combustor, a gas turbine and an exhaust port. The heat exchanger is positioned within the air intake duct. The heat exchange fluid flows through the heat exchanger and supplies a chilled refrigerant stream for cooling and densifying the air intake stream which then flows into the air compressor.
A waste-heat boiler is downstream of and in communication with the exhaust port of the gas turbine. The exhaust of the gas turbine converts a stream of water flowing through the boiler into high pressure steam.
The steam turbine plant comprises a steam turbine and a condenser for spent steam. The high pressure steam from the boiler is used to drive the steam turbine. The spent steam from the turbine flows into a condenser. The heat exchange fluid flows through the condenser and condenses the spent steam. The heat exchange fluid then returns and flows through the regasifier/chiller in the LNG fuel supply system.
In the other embodiment of the invention, the heat exchange fluid, a water/glycol mixture, flows through the regasifier/chiller (heat exchanger) in the LNG fuel supply system. The LNG chills the heat exchange fluid which then flows through a heat exchanger in the gas turbine plant. The gas turbine plant, which is fueled by the gasified LNG, drives a generator. The gas turbine plant has an air intake duct, the heat exchanger, a water separator, an air compressor, a combustor, a gas turbine and an exhaust port. The heat exchanger is positioned within the air intake duct. The primary heat exchange fluid flows through the heat exchanger and supplies a chilled refrigerant stream for cooling and densifying the air intake stream to the air compressor.
A heat recovery heat exchanger is downstream of and in communication with the exhaust port of the gas turbine. The heat exchange fluid flows through the heat recovery heat exchanger. The heat exchange fluid then returns and flows through the regasifier/chiller in the LNG fuel supply system.